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dl |
1.1 |
/* |
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jsr166 |
1.66 |
* Written by Doug Lea and Martin Buchholz with assistance from members of |
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* JCP JSR-166 Expert Group and released to the public domain, as explained |
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jsr166 |
1.73 |
* at http://creativecommons.org/publicdomain/zero/1.0/ |
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dl |
1.1 |
*/ |
6 |
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package java.util.concurrent; |
8 |
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9 |
dl |
1.123 |
import java.lang.invoke.MethodHandles; |
10 |
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import java.lang.invoke.VarHandle; |
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jsr166 |
1.51 |
import java.util.AbstractQueue; |
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jsr166 |
1.117 |
import java.util.Arrays; |
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jsr166 |
1.51 |
import java.util.Collection; |
14 |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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jsr166 |
1.118 |
import java.util.Objects; |
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jsr166 |
1.51 |
import java.util.Queue; |
18 |
dl |
1.82 |
import java.util.Spliterator; |
19 |
dl |
1.83 |
import java.util.Spliterators; |
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jsr166 |
1.106 |
import java.util.function.Consumer; |
21 |
dl |
1.1 |
|
22 |
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/** |
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jsr166 |
1.29 |
* An unbounded thread-safe {@linkplain Queue queue} based on linked nodes. |
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dholmes |
1.6 |
* This queue orders elements FIFO (first-in-first-out). |
25 |
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* The <em>head</em> of the queue is that element that has been on the |
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* queue the longest time. |
27 |
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* The <em>tail</em> of the queue is that element that has been on the |
28 |
dl |
1.17 |
* queue the shortest time. New elements |
29 |
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* are inserted at the tail of the queue, and the queue retrieval |
30 |
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* operations obtain elements at the head of the queue. |
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jsr166 |
1.48 |
* A {@code ConcurrentLinkedQueue} is an appropriate choice when |
32 |
dl |
1.19 |
* many threads will share access to a common collection. |
33 |
jsr166 |
1.55 |
* Like most other concurrent collection implementations, this class |
34 |
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* does not permit the use of {@code null} elements. |
35 |
dl |
1.1 |
* |
36 |
jsr166 |
1.93 |
* <p>This implementation employs an efficient <em>non-blocking</em> |
37 |
jsr166 |
1.99 |
* algorithm based on one described in |
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* <a href="http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf"> |
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* Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue |
40 |
dl |
1.15 |
* Algorithms</a> by Maged M. Michael and Michael L. Scott. |
41 |
dl |
1.1 |
* |
42 |
jsr166 |
1.55 |
* <p>Iterators are <i>weakly consistent</i>, returning elements |
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* reflecting the state of the queue at some point at or since the |
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* creation of the iterator. They do <em>not</em> throw {@link |
45 |
dl |
1.68 |
* java.util.ConcurrentModificationException}, and may proceed concurrently |
46 |
jsr166 |
1.69 |
* with other operations. Elements contained in the queue since the creation |
47 |
jsr166 |
1.55 |
* of the iterator will be returned exactly once. |
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* |
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* <p>Beware that, unlike in most collections, the {@code size} method |
50 |
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* is <em>NOT</em> a constant-time operation. Because of the |
51 |
dl |
1.1 |
* asynchronous nature of these queues, determining the current number |
52 |
dl |
1.74 |
* of elements requires a traversal of the elements, and so may report |
53 |
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* inaccurate results if this collection is modified during traversal. |
54 |
dl |
1.75 |
* Additionally, the bulk operations {@code addAll}, |
55 |
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* {@code removeAll}, {@code retainAll}, {@code containsAll}, |
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jsr166 |
1.126 |
* and {@code toArray} are <em>not</em> guaranteed |
57 |
dl |
1.74 |
* to be performed atomically. For example, an iterator operating |
58 |
dl |
1.75 |
* concurrently with an {@code addAll} operation might view only some |
59 |
dl |
1.74 |
* of the added elements. |
60 |
dl |
1.18 |
* |
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jsr166 |
1.55 |
* <p>This class and its iterator implement all of the <em>optional</em> |
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* methods of the {@link Queue} and {@link Iterator} interfaces. |
63 |
dl |
1.18 |
* |
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jsr166 |
1.43 |
* <p>Memory consistency effects: As with other concurrent |
65 |
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* collections, actions in a thread prior to placing an object into a |
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* {@code ConcurrentLinkedQueue} |
67 |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
68 |
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* actions subsequent to the access or removal of that element from |
69 |
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* the {@code ConcurrentLinkedQueue} in another thread. |
70 |
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* |
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dl |
1.25 |
* <p>This class is a member of the |
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jsr166 |
1.47 |
* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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dl |
1.25 |
* Java Collections Framework</a>. |
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* |
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dl |
1.1 |
* @since 1.5 |
76 |
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* @author Doug Lea |
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jsr166 |
1.105 |
* @param <E> the type of elements held in this queue |
78 |
dl |
1.25 |
*/ |
79 |
dl |
1.1 |
public class ConcurrentLinkedQueue<E> extends AbstractQueue<E> |
80 |
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implements Queue<E>, java.io.Serializable { |
81 |
dl |
1.14 |
private static final long serialVersionUID = 196745693267521676L; |
82 |
dl |
1.1 |
|
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/* |
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jsr166 |
1.48 |
* This is a modification of the Michael & Scott algorithm, |
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* adapted for a garbage-collected environment, with support for |
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* interior node deletion (to support remove(Object)). For |
87 |
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* explanation, read the paper. |
88 |
dl |
1.44 |
* |
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jsr166 |
1.48 |
* Note that like most non-blocking algorithms in this package, |
90 |
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* this implementation relies on the fact that in garbage |
91 |
dl |
1.44 |
* collected systems, there is no possibility of ABA problems due |
92 |
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* to recycled nodes, so there is no need to use "counted |
93 |
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* pointers" or related techniques seen in versions used in |
94 |
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* non-GC'ed settings. |
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jsr166 |
1.48 |
* |
96 |
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* The fundamental invariants are: |
97 |
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* - There is exactly one (last) Node with a null next reference, |
98 |
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* which is CASed when enqueueing. This last Node can be |
99 |
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* reached in O(1) time from tail, but tail is merely an |
100 |
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* optimization - it can always be reached in O(N) time from |
101 |
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* head as well. |
102 |
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* - The elements contained in the queue are the non-null items in |
103 |
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* Nodes that are reachable from head. CASing the item |
104 |
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* reference of a Node to null atomically removes it from the |
105 |
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* queue. Reachability of all elements from head must remain |
106 |
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* true even in the case of concurrent modifications that cause |
107 |
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* head to advance. A dequeued Node may remain in use |
108 |
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* indefinitely due to creation of an Iterator or simply a |
109 |
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* poll() that has lost its time slice. |
110 |
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* |
111 |
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* The above might appear to imply that all Nodes are GC-reachable |
112 |
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* from a predecessor dequeued Node. That would cause two problems: |
113 |
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* - allow a rogue Iterator to cause unbounded memory retention |
114 |
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* - cause cross-generational linking of old Nodes to new Nodes if |
115 |
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* a Node was tenured while live, which generational GCs have a |
116 |
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* hard time dealing with, causing repeated major collections. |
117 |
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* However, only non-deleted Nodes need to be reachable from |
118 |
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* dequeued Nodes, and reachability does not necessarily have to |
119 |
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* be of the kind understood by the GC. We use the trick of |
120 |
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* linking a Node that has just been dequeued to itself. Such a |
121 |
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* self-link implicitly means to advance to head. |
122 |
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* |
123 |
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* Both head and tail are permitted to lag. In fact, failing to |
124 |
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* update them every time one could is a significant optimization |
125 |
jsr166 |
1.65 |
* (fewer CASes). As with LinkedTransferQueue (see the internal |
126 |
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* documentation for that class), we use a slack threshold of two; |
127 |
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* that is, we update head/tail when the current pointer appears |
128 |
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* to be two or more steps away from the first/last node. |
129 |
jsr166 |
1.48 |
* |
130 |
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* Since head and tail are updated concurrently and independently, |
131 |
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* it is possible for tail to lag behind head (why not)? |
132 |
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* |
133 |
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* CASing a Node's item reference to null atomically removes the |
134 |
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* element from the queue. Iterators skip over Nodes with null |
135 |
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* items. Prior implementations of this class had a race between |
136 |
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* poll() and remove(Object) where the same element would appear |
137 |
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* to be successfully removed by two concurrent operations. The |
138 |
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* method remove(Object) also lazily unlinks deleted Nodes, but |
139 |
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* this is merely an optimization. |
140 |
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* |
141 |
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* When constructing a Node (before enqueuing it) we avoid paying |
142 |
jsr166 |
1.124 |
* for a volatile write to item. This allows the cost of enqueue |
143 |
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* to be "one-and-a-half" CASes. |
144 |
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1.48 |
* |
145 |
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* Both head and tail may or may not point to a Node with a |
146 |
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* non-null item. If the queue is empty, all items must of course |
147 |
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* be null. Upon creation, both head and tail refer to a dummy |
148 |
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* Node with null item. Both head and tail are only updated using |
149 |
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* CAS, so they never regress, although again this is merely an |
150 |
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* optimization. |
151 |
dl |
1.1 |
*/ |
152 |
jsr166 |
1.51 |
|
153 |
dl |
1.123 |
static final class Node<E> { |
154 |
jsr166 |
1.64 |
volatile E item; |
155 |
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volatile Node<E> next; |
156 |
jsr166 |
1.102 |
} |
157 |
jsr166 |
1.29 |
|
158 |
jsr166 |
1.102 |
/** |
159 |
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* Returns a new node holding item. Uses relaxed write because item |
160 |
dl |
1.123 |
* can only be seen after piggy-backing publication via CAS. |
161 |
jsr166 |
1.102 |
*/ |
162 |
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static <E> Node<E> newNode(E item) { |
163 |
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Node<E> node = new Node<E>(); |
164 |
dl |
1.123 |
ITEM.set(node, item); |
165 |
jsr166 |
1.102 |
return node; |
166 |
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} |
167 |
jsr166 |
1.29 |
|
168 |
tim |
1.2 |
/** |
169 |
jsr166 |
1.51 |
* A node from which the first live (non-deleted) node (if any) |
170 |
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* can be reached in O(1) time. |
171 |
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* Invariants: |
172 |
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* - all live nodes are reachable from head via succ() |
173 |
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* - head != null |
174 |
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* - (tmp = head).next != tmp || tmp != head |
175 |
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* Non-invariants: |
176 |
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* - head.item may or may not be null. |
177 |
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* - it is permitted for tail to lag behind head, that is, for tail |
178 |
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* to not be reachable from head! |
179 |
dl |
1.1 |
*/ |
180 |
jsr166 |
1.114 |
transient volatile Node<E> head; |
181 |
dl |
1.1 |
|
182 |
jsr166 |
1.51 |
/** |
183 |
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* A node from which the last node on list (that is, the unique |
184 |
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* node with node.next == null) can be reached in O(1) time. |
185 |
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* Invariants: |
186 |
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* - the last node is always reachable from tail via succ() |
187 |
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* - tail != null |
188 |
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* Non-invariants: |
189 |
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* - tail.item may or may not be null. |
190 |
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* - it is permitted for tail to lag behind head, that is, for tail |
191 |
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* to not be reachable from head! |
192 |
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* - tail.next may or may not be self-pointing to tail. |
193 |
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*/ |
194 |
jsr166 |
1.55 |
private transient volatile Node<E> tail; |
195 |
dl |
1.1 |
|
196 |
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/** |
197 |
jsr166 |
1.48 |
* Creates a {@code ConcurrentLinkedQueue} that is initially empty. |
198 |
dl |
1.1 |
*/ |
199 |
jsr166 |
1.55 |
public ConcurrentLinkedQueue() { |
200 |
jsr166 |
1.102 |
head = tail = newNode(null); |
201 |
jsr166 |
1.55 |
} |
202 |
dl |
1.1 |
|
203 |
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/** |
204 |
jsr166 |
1.48 |
* Creates a {@code ConcurrentLinkedQueue} |
205 |
dholmes |
1.7 |
* initially containing the elements of the given collection, |
206 |
dholmes |
1.6 |
* added in traversal order of the collection's iterator. |
207 |
jsr166 |
1.55 |
* |
208 |
dholmes |
1.6 |
* @param c the collection of elements to initially contain |
209 |
jsr166 |
1.34 |
* @throws NullPointerException if the specified collection or any |
210 |
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* of its elements are null |
211 |
dl |
1.1 |
*/ |
212 |
dholmes |
1.6 |
public ConcurrentLinkedQueue(Collection<? extends E> c) { |
213 |
jsr166 |
1.55 |
Node<E> h = null, t = null; |
214 |
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for (E e : c) { |
215 |
jsr166 |
1.118 |
Node<E> newNode = newNode(Objects.requireNonNull(e)); |
216 |
jsr166 |
1.55 |
if (h == null) |
217 |
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h = t = newNode; |
218 |
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else { |
219 |
jsr166 |
1.125 |
NEXT.set(t, newNode); |
220 |
jsr166 |
1.55 |
t = newNode; |
221 |
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} |
222 |
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} |
223 |
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if (h == null) |
224 |
jsr166 |
1.102 |
h = t = newNode(null); |
225 |
jsr166 |
1.55 |
head = h; |
226 |
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tail = t; |
227 |
dl |
1.1 |
} |
228 |
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229 |
jsr166 |
1.29 |
// Have to override just to update the javadoc |
230 |
dholmes |
1.6 |
|
231 |
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/** |
232 |
jsr166 |
1.35 |
* Inserts the specified element at the tail of this queue. |
233 |
jsr166 |
1.67 |
* As the queue is unbounded, this method will never throw |
234 |
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* {@link IllegalStateException} or return {@code false}. |
235 |
dholmes |
1.7 |
* |
236 |
jsr166 |
1.48 |
* @return {@code true} (as specified by {@link Collection#add}) |
237 |
jsr166 |
1.32 |
* @throws NullPointerException if the specified element is null |
238 |
dholmes |
1.6 |
*/ |
239 |
jsr166 |
1.31 |
public boolean add(E e) { |
240 |
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return offer(e); |
241 |
dholmes |
1.6 |
} |
242 |
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243 |
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/** |
244 |
jsr166 |
1.81 |
* Tries to CAS head to p. If successful, repoint old head to itself |
245 |
jsr166 |
1.48 |
* as sentinel for succ(), below. |
246 |
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*/ |
247 |
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final void updateHead(Node<E> h, Node<E> p) { |
248 |
jsr166 |
1.113 |
// assert h != null && p != null && (h == p || h.item == null); |
249 |
dl |
1.123 |
if (h != p && HEAD.compareAndSet(this, h, p)) |
250 |
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NEXT.setRelease(h, h); |
251 |
jsr166 |
1.48 |
} |
252 |
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253 |
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/** |
254 |
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* Returns the successor of p, or the head node if p.next has been |
255 |
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* linked to self, which will only be true if traversing with a |
256 |
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* stale pointer that is now off the list. |
257 |
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*/ |
258 |
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final Node<E> succ(Node<E> p) { |
259 |
jsr166 |
1.55 |
Node<E> next = p.next; |
260 |
jsr166 |
1.48 |
return (p == next) ? head : next; |
261 |
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} |
262 |
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263 |
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/** |
264 |
jsr166 |
1.32 |
* Inserts the specified element at the tail of this queue. |
265 |
jsr166 |
1.67 |
* As the queue is unbounded, this method will never return {@code false}. |
266 |
dl |
1.17 |
* |
267 |
jsr166 |
1.48 |
* @return {@code true} (as specified by {@link Queue#offer}) |
268 |
jsr166 |
1.32 |
* @throws NullPointerException if the specified element is null |
269 |
dholmes |
1.6 |
*/ |
270 |
jsr166 |
1.31 |
public boolean offer(E e) { |
271 |
jsr166 |
1.118 |
final Node<E> newNode = newNode(Objects.requireNonNull(e)); |
272 |
jsr166 |
1.58 |
|
273 |
jsr166 |
1.65 |
for (Node<E> t = tail, p = t;;) { |
274 |
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Node<E> q = p.next; |
275 |
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if (q == null) { |
276 |
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// p is last node |
277 |
dl |
1.123 |
if (NEXT.compareAndSet(p, null, newNode)) { |
278 |
jsr166 |
1.63 |
// Successful CAS is the linearization point |
279 |
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// for e to become an element of this queue, |
280 |
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// and for newNode to become "live". |
281 |
jsr166 |
1.128 |
if (p != t) // hop two nodes at a time; failure is OK |
282 |
jsr166 |
1.129 |
TAIL.weakCompareAndSet(this, t, newNode); |
283 |
jsr166 |
1.48 |
return true; |
284 |
dl |
1.1 |
} |
285 |
jsr166 |
1.65 |
// Lost CAS race to another thread; re-read next |
286 |
dl |
1.1 |
} |
287 |
jsr166 |
1.65 |
else if (p == q) |
288 |
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// We have fallen off list. If tail is unchanged, it |
289 |
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// will also be off-list, in which case we need to |
290 |
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// jump to head, from which all live nodes are always |
291 |
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// reachable. Else the new tail is a better bet. |
292 |
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p = (t != (t = tail)) ? t : head; |
293 |
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else |
294 |
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// Check for tail updates after two hops. |
295 |
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p = (p != t && t != (t = tail)) ? t : q; |
296 |
dl |
1.1 |
} |
297 |
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} |
298 |
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|
299 |
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public E poll() { |
300 |
jsr166 |
1.65 |
restartFromHead: |
301 |
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for (;;) { |
302 |
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for (Node<E> h = head, p = h, q;;) { |
303 |
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E item = p.item; |
304 |
jsr166 |
1.48 |
|
305 |
dl |
1.123 |
if (item != null && ITEM.compareAndSet(p, item, null)) { |
306 |
jsr166 |
1.65 |
// Successful CAS is the linearization point |
307 |
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// for item to be removed from this queue. |
308 |
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if (p != h) // hop two nodes at a time |
309 |
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updateHead(h, ((q = p.next) != null) ? q : p); |
310 |
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return item; |
311 |
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} |
312 |
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else if ((q = p.next) == null) { |
313 |
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updateHead(h, p); |
314 |
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return null; |
315 |
dl |
1.1 |
} |
316 |
jsr166 |
1.65 |
else if (p == q) |
317 |
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continue restartFromHead; |
318 |
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else |
319 |
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p = q; |
320 |
dl |
1.1 |
} |
321 |
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} |
322 |
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} |
323 |
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|
324 |
jsr166 |
1.48 |
public E peek() { |
325 |
jsr166 |
1.65 |
restartFromHead: |
326 |
dl |
1.1 |
for (;;) { |
327 |
jsr166 |
1.65 |
for (Node<E> h = head, p = h, q;;) { |
328 |
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E item = p.item; |
329 |
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if (item != null || (q = p.next) == null) { |
330 |
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updateHead(h, p); |
331 |
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return item; |
332 |
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} |
333 |
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else if (p == q) |
334 |
|
|
continue restartFromHead; |
335 |
|
|
else |
336 |
|
|
p = q; |
337 |
dl |
1.1 |
} |
338 |
|
|
} |
339 |
|
|
} |
340 |
|
|
|
341 |
|
|
/** |
342 |
jsr166 |
1.51 |
* Returns the first live (non-deleted) node on list, or null if none. |
343 |
|
|
* This is yet another variant of poll/peek; here returning the |
344 |
|
|
* first node, not element. We could make peek() a wrapper around |
345 |
|
|
* first(), but that would cost an extra volatile read of item, |
346 |
|
|
* and the need to add a retry loop to deal with the possibility |
347 |
|
|
* of losing a race to a concurrent poll(). |
348 |
dl |
1.1 |
*/ |
349 |
dl |
1.23 |
Node<E> first() { |
350 |
jsr166 |
1.65 |
restartFromHead: |
351 |
dl |
1.1 |
for (;;) { |
352 |
jsr166 |
1.65 |
for (Node<E> h = head, p = h, q;;) { |
353 |
|
|
boolean hasItem = (p.item != null); |
354 |
|
|
if (hasItem || (q = p.next) == null) { |
355 |
|
|
updateHead(h, p); |
356 |
|
|
return hasItem ? p : null; |
357 |
|
|
} |
358 |
|
|
else if (p == q) |
359 |
|
|
continue restartFromHead; |
360 |
|
|
else |
361 |
|
|
p = q; |
362 |
dl |
1.1 |
} |
363 |
|
|
} |
364 |
|
|
} |
365 |
|
|
|
366 |
dl |
1.28 |
/** |
367 |
jsr166 |
1.48 |
* Returns {@code true} if this queue contains no elements. |
368 |
dl |
1.28 |
* |
369 |
jsr166 |
1.48 |
* @return {@code true} if this queue contains no elements |
370 |
dl |
1.28 |
*/ |
371 |
dl |
1.1 |
public boolean isEmpty() { |
372 |
|
|
return first() == null; |
373 |
|
|
} |
374 |
|
|
|
375 |
|
|
/** |
376 |
dl |
1.17 |
* Returns the number of elements in this queue. If this queue |
377 |
jsr166 |
1.48 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
378 |
|
|
* {@code Integer.MAX_VALUE}. |
379 |
tim |
1.2 |
* |
380 |
dl |
1.17 |
* <p>Beware that, unlike in most collections, this method is |
381 |
dl |
1.1 |
* <em>NOT</em> a constant-time operation. Because of the |
382 |
|
|
* asynchronous nature of these queues, determining the current |
383 |
jsr166 |
1.55 |
* number of elements requires an O(n) traversal. |
384 |
|
|
* Additionally, if elements are added or removed during execution |
385 |
|
|
* of this method, the returned result may be inaccurate. Thus, |
386 |
|
|
* this method is typically not very useful in concurrent |
387 |
|
|
* applications. |
388 |
dl |
1.17 |
* |
389 |
jsr166 |
1.37 |
* @return the number of elements in this queue |
390 |
tim |
1.2 |
*/ |
391 |
dl |
1.1 |
public int size() { |
392 |
jsr166 |
1.100 |
restartFromHead: for (;;) { |
393 |
|
|
int count = 0; |
394 |
|
|
for (Node<E> p = first(); p != null;) { |
395 |
|
|
if (p.item != null) |
396 |
|
|
if (++count == Integer.MAX_VALUE) |
397 |
|
|
break; // @see Collection.size() |
398 |
jsr166 |
1.116 |
if (p == (p = p.next)) |
399 |
jsr166 |
1.100 |
continue restartFromHead; |
400 |
|
|
} |
401 |
|
|
return count; |
402 |
|
|
} |
403 |
dl |
1.1 |
} |
404 |
|
|
|
405 |
jsr166 |
1.37 |
/** |
406 |
jsr166 |
1.48 |
* Returns {@code true} if this queue contains the specified element. |
407 |
|
|
* More formally, returns {@code true} if and only if this queue contains |
408 |
|
|
* at least one element {@code e} such that {@code o.equals(e)}. |
409 |
jsr166 |
1.37 |
* |
410 |
|
|
* @param o object to be checked for containment in this queue |
411 |
jsr166 |
1.48 |
* @return {@code true} if this queue contains the specified element |
412 |
jsr166 |
1.37 |
*/ |
413 |
dholmes |
1.6 |
public boolean contains(Object o) { |
414 |
jsr166 |
1.103 |
if (o != null) { |
415 |
|
|
for (Node<E> p = first(); p != null; p = succ(p)) { |
416 |
|
|
E item = p.item; |
417 |
|
|
if (item != null && o.equals(item)) |
418 |
|
|
return true; |
419 |
|
|
} |
420 |
dl |
1.1 |
} |
421 |
|
|
return false; |
422 |
|
|
} |
423 |
|
|
|
424 |
jsr166 |
1.37 |
/** |
425 |
|
|
* Removes a single instance of the specified element from this queue, |
426 |
jsr166 |
1.48 |
* if it is present. More formally, removes an element {@code e} such |
427 |
|
|
* that {@code o.equals(e)}, if this queue contains one or more such |
428 |
jsr166 |
1.37 |
* elements. |
429 |
jsr166 |
1.48 |
* Returns {@code true} if this queue contained the specified element |
430 |
jsr166 |
1.37 |
* (or equivalently, if this queue changed as a result of the call). |
431 |
|
|
* |
432 |
|
|
* @param o element to be removed from this queue, if present |
433 |
jsr166 |
1.48 |
* @return {@code true} if this queue changed as a result of the call |
434 |
jsr166 |
1.37 |
*/ |
435 |
dholmes |
1.6 |
public boolean remove(Object o) { |
436 |
jsr166 |
1.103 |
if (o != null) { |
437 |
jsr166 |
1.110 |
Node<E> next, pred = null; |
438 |
|
|
for (Node<E> p = first(); p != null; pred = p, p = next) { |
439 |
|
|
boolean removed = false; |
440 |
jsr166 |
1.103 |
E item = p.item; |
441 |
jsr166 |
1.110 |
if (item != null) { |
442 |
|
|
if (!o.equals(item)) { |
443 |
|
|
next = succ(p); |
444 |
|
|
continue; |
445 |
|
|
} |
446 |
dl |
1.123 |
removed = ITEM.compareAndSet(p, item, null); |
447 |
jsr166 |
1.110 |
} |
448 |
|
|
|
449 |
|
|
next = succ(p); |
450 |
|
|
if (pred != null && next != null) // unlink |
451 |
jsr166 |
1.129 |
NEXT.weakCompareAndSet(pred, p, next); |
452 |
jsr166 |
1.110 |
if (removed) |
453 |
jsr166 |
1.103 |
return true; |
454 |
jsr166 |
1.48 |
} |
455 |
dl |
1.1 |
} |
456 |
|
|
return false; |
457 |
|
|
} |
458 |
tim |
1.2 |
|
459 |
jsr166 |
1.33 |
/** |
460 |
jsr166 |
1.55 |
* Appends all of the elements in the specified collection to the end of |
461 |
|
|
* this queue, in the order that they are returned by the specified |
462 |
jsr166 |
1.56 |
* collection's iterator. Attempts to {@code addAll} of a queue to |
463 |
|
|
* itself result in {@code IllegalArgumentException}. |
464 |
jsr166 |
1.55 |
* |
465 |
|
|
* @param c the elements to be inserted into this queue |
466 |
|
|
* @return {@code true} if this queue changed as a result of the call |
467 |
jsr166 |
1.56 |
* @throws NullPointerException if the specified collection or any |
468 |
|
|
* of its elements are null |
469 |
|
|
* @throws IllegalArgumentException if the collection is this queue |
470 |
jsr166 |
1.55 |
*/ |
471 |
|
|
public boolean addAll(Collection<? extends E> c) { |
472 |
jsr166 |
1.56 |
if (c == this) |
473 |
|
|
// As historically specified in AbstractQueue#addAll |
474 |
|
|
throw new IllegalArgumentException(); |
475 |
|
|
|
476 |
jsr166 |
1.55 |
// Copy c into a private chain of Nodes |
477 |
jsr166 |
1.65 |
Node<E> beginningOfTheEnd = null, last = null; |
478 |
jsr166 |
1.55 |
for (E e : c) { |
479 |
jsr166 |
1.118 |
Node<E> newNode = newNode(Objects.requireNonNull(e)); |
480 |
jsr166 |
1.65 |
if (beginningOfTheEnd == null) |
481 |
|
|
beginningOfTheEnd = last = newNode; |
482 |
jsr166 |
1.55 |
else { |
483 |
jsr166 |
1.125 |
NEXT.set(last, newNode); |
484 |
jsr166 |
1.55 |
last = newNode; |
485 |
|
|
} |
486 |
|
|
} |
487 |
jsr166 |
1.65 |
if (beginningOfTheEnd == null) |
488 |
jsr166 |
1.55 |
return false; |
489 |
|
|
|
490 |
jsr166 |
1.65 |
// Atomically append the chain at the tail of this collection |
491 |
|
|
for (Node<E> t = tail, p = t;;) { |
492 |
|
|
Node<E> q = p.next; |
493 |
|
|
if (q == null) { |
494 |
|
|
// p is last node |
495 |
dl |
1.123 |
if (NEXT.compareAndSet(p, null, beginningOfTheEnd)) { |
496 |
jsr166 |
1.65 |
// Successful CAS is the linearization point |
497 |
|
|
// for all elements to be added to this queue. |
498 |
jsr166 |
1.129 |
if (!TAIL.weakCompareAndSet(this, t, last)) { |
499 |
jsr166 |
1.55 |
// Try a little harder to update tail, |
500 |
|
|
// since we may be adding many elements. |
501 |
|
|
t = tail; |
502 |
|
|
if (last.next == null) |
503 |
jsr166 |
1.129 |
TAIL.weakCompareAndSet(this, t, last); |
504 |
jsr166 |
1.55 |
} |
505 |
|
|
return true; |
506 |
|
|
} |
507 |
jsr166 |
1.65 |
// Lost CAS race to another thread; re-read next |
508 |
jsr166 |
1.55 |
} |
509 |
jsr166 |
1.65 |
else if (p == q) |
510 |
|
|
// We have fallen off list. If tail is unchanged, it |
511 |
|
|
// will also be off-list, in which case we need to |
512 |
|
|
// jump to head, from which all live nodes are always |
513 |
|
|
// reachable. Else the new tail is a better bet. |
514 |
|
|
p = (t != (t = tail)) ? t : head; |
515 |
|
|
else |
516 |
|
|
// Check for tail updates after two hops. |
517 |
|
|
p = (p != t && t != (t = tail)) ? t : q; |
518 |
jsr166 |
1.55 |
} |
519 |
|
|
} |
520 |
|
|
|
521 |
jsr166 |
1.117 |
public String toString() { |
522 |
|
|
String[] a = null; |
523 |
|
|
restartFromHead: for (;;) { |
524 |
|
|
int charLength = 0; |
525 |
|
|
int size = 0; |
526 |
|
|
for (Node<E> p = first(); p != null;) { |
527 |
|
|
E item = p.item; |
528 |
|
|
if (item != null) { |
529 |
|
|
if (a == null) |
530 |
|
|
a = new String[4]; |
531 |
|
|
else if (size == a.length) |
532 |
|
|
a = Arrays.copyOf(a, 2 * size); |
533 |
|
|
String s = item.toString(); |
534 |
|
|
a[size++] = s; |
535 |
|
|
charLength += s.length(); |
536 |
|
|
} |
537 |
|
|
if (p == (p = p.next)) |
538 |
|
|
continue restartFromHead; |
539 |
|
|
} |
540 |
|
|
|
541 |
|
|
if (size == 0) |
542 |
|
|
return "[]"; |
543 |
|
|
|
544 |
jsr166 |
1.120 |
return Helpers.toString(a, size, charLength); |
545 |
jsr166 |
1.117 |
} |
546 |
|
|
} |
547 |
|
|
|
548 |
|
|
private Object[] toArrayInternal(Object[] a) { |
549 |
|
|
Object[] x = a; |
550 |
|
|
restartFromHead: for (;;) { |
551 |
|
|
int size = 0; |
552 |
|
|
for (Node<E> p = first(); p != null;) { |
553 |
|
|
E item = p.item; |
554 |
|
|
if (item != null) { |
555 |
|
|
if (x == null) |
556 |
|
|
x = new Object[4]; |
557 |
|
|
else if (size == x.length) |
558 |
|
|
x = Arrays.copyOf(x, 2 * (size + 4)); |
559 |
|
|
x[size++] = item; |
560 |
|
|
} |
561 |
|
|
if (p == (p = p.next)) |
562 |
|
|
continue restartFromHead; |
563 |
|
|
} |
564 |
|
|
if (x == null) |
565 |
|
|
return new Object[0]; |
566 |
|
|
else if (a != null && size <= a.length) { |
567 |
|
|
if (a != x) |
568 |
|
|
System.arraycopy(x, 0, a, 0, size); |
569 |
|
|
if (size < a.length) |
570 |
|
|
a[size] = null; |
571 |
|
|
return a; |
572 |
|
|
} |
573 |
|
|
return (size == x.length) ? x : Arrays.copyOf(x, size); |
574 |
|
|
} |
575 |
|
|
} |
576 |
|
|
|
577 |
jsr166 |
1.55 |
/** |
578 |
jsr166 |
1.48 |
* Returns an array containing all of the elements in this queue, in |
579 |
|
|
* proper sequence. |
580 |
|
|
* |
581 |
|
|
* <p>The returned array will be "safe" in that no references to it are |
582 |
|
|
* maintained by this queue. (In other words, this method must allocate |
583 |
|
|
* a new array). The caller is thus free to modify the returned array. |
584 |
|
|
* |
585 |
|
|
* <p>This method acts as bridge between array-based and collection-based |
586 |
|
|
* APIs. |
587 |
|
|
* |
588 |
|
|
* @return an array containing all of the elements in this queue |
589 |
|
|
*/ |
590 |
|
|
public Object[] toArray() { |
591 |
jsr166 |
1.117 |
return toArrayInternal(null); |
592 |
jsr166 |
1.48 |
} |
593 |
|
|
|
594 |
|
|
/** |
595 |
|
|
* Returns an array containing all of the elements in this queue, in |
596 |
|
|
* proper sequence; the runtime type of the returned array is that of |
597 |
|
|
* the specified array. If the queue fits in the specified array, it |
598 |
|
|
* is returned therein. Otherwise, a new array is allocated with the |
599 |
|
|
* runtime type of the specified array and the size of this queue. |
600 |
|
|
* |
601 |
|
|
* <p>If this queue fits in the specified array with room to spare |
602 |
|
|
* (i.e., the array has more elements than this queue), the element in |
603 |
|
|
* the array immediately following the end of the queue is set to |
604 |
|
|
* {@code null}. |
605 |
|
|
* |
606 |
|
|
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
607 |
|
|
* array-based and collection-based APIs. Further, this method allows |
608 |
|
|
* precise control over the runtime type of the output array, and may, |
609 |
|
|
* under certain circumstances, be used to save allocation costs. |
610 |
|
|
* |
611 |
|
|
* <p>Suppose {@code x} is a queue known to contain only strings. |
612 |
|
|
* The following code can be used to dump the queue into a newly |
613 |
|
|
* allocated array of {@code String}: |
614 |
|
|
* |
615 |
jsr166 |
1.115 |
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
616 |
jsr166 |
1.48 |
* |
617 |
|
|
* Note that {@code toArray(new Object[0])} is identical in function to |
618 |
|
|
* {@code toArray()}. |
619 |
|
|
* |
620 |
|
|
* @param a the array into which the elements of the queue are to |
621 |
|
|
* be stored, if it is big enough; otherwise, a new array of the |
622 |
|
|
* same runtime type is allocated for this purpose |
623 |
|
|
* @return an array containing all of the elements in this queue |
624 |
|
|
* @throws ArrayStoreException if the runtime type of the specified array |
625 |
|
|
* is not a supertype of the runtime type of every element in |
626 |
|
|
* this queue |
627 |
|
|
* @throws NullPointerException if the specified array is null |
628 |
|
|
*/ |
629 |
|
|
@SuppressWarnings("unchecked") |
630 |
|
|
public <T> T[] toArray(T[] a) { |
631 |
jsr166 |
1.117 |
if (a == null) throw new NullPointerException(); |
632 |
|
|
return (T[]) toArrayInternal(a); |
633 |
jsr166 |
1.48 |
} |
634 |
|
|
|
635 |
|
|
/** |
636 |
dholmes |
1.7 |
* Returns an iterator over the elements in this queue in proper sequence. |
637 |
jsr166 |
1.55 |
* The elements will be returned in order from first (head) to last (tail). |
638 |
|
|
* |
639 |
jsr166 |
1.98 |
* <p>The returned iterator is |
640 |
|
|
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
641 |
dholmes |
1.7 |
* |
642 |
jsr166 |
1.33 |
* @return an iterator over the elements in this queue in proper sequence |
643 |
dholmes |
1.7 |
*/ |
644 |
dl |
1.1 |
public Iterator<E> iterator() { |
645 |
|
|
return new Itr(); |
646 |
|
|
} |
647 |
|
|
|
648 |
|
|
private class Itr implements Iterator<E> { |
649 |
|
|
/** |
650 |
|
|
* Next node to return item for. |
651 |
|
|
*/ |
652 |
dl |
1.23 |
private Node<E> nextNode; |
653 |
dl |
1.1 |
|
654 |
tim |
1.2 |
/** |
655 |
dl |
1.1 |
* nextItem holds on to item fields because once we claim |
656 |
|
|
* that an element exists in hasNext(), we must return it in |
657 |
|
|
* the following next() call even if it was in the process of |
658 |
|
|
* being removed when hasNext() was called. |
659 |
jsr166 |
1.29 |
*/ |
660 |
dl |
1.1 |
private E nextItem; |
661 |
|
|
|
662 |
|
|
/** |
663 |
|
|
* Node of the last returned item, to support remove. |
664 |
|
|
*/ |
665 |
dl |
1.23 |
private Node<E> lastRet; |
666 |
dl |
1.1 |
|
667 |
tim |
1.2 |
Itr() { |
668 |
jsr166 |
1.114 |
restartFromHead: for (;;) { |
669 |
|
|
Node<E> h, p, q; |
670 |
|
|
for (p = h = head;; p = q) { |
671 |
|
|
E item; |
672 |
|
|
if ((item = p.item) != null) { |
673 |
|
|
nextNode = p; |
674 |
|
|
nextItem = item; |
675 |
|
|
break; |
676 |
|
|
} |
677 |
|
|
else if ((q = p.next) == null) |
678 |
|
|
break; |
679 |
|
|
else if (p == q) |
680 |
|
|
continue restartFromHead; |
681 |
|
|
} |
682 |
|
|
updateHead(h, p); |
683 |
|
|
return; |
684 |
|
|
} |
685 |
dl |
1.1 |
} |
686 |
tim |
1.2 |
|
687 |
jsr166 |
1.114 |
public boolean hasNext() { |
688 |
|
|
return nextItem != null; |
689 |
|
|
} |
690 |
dl |
1.1 |
|
691 |
jsr166 |
1.114 |
public E next() { |
692 |
jsr166 |
1.111 |
final Node<E> pred = nextNode; |
693 |
jsr166 |
1.114 |
if (pred == null) throw new NoSuchElementException(); |
694 |
|
|
// assert nextItem != null; |
695 |
|
|
lastRet = pred; |
696 |
|
|
E item = null; |
697 |
jsr166 |
1.48 |
|
698 |
jsr166 |
1.114 |
for (Node<E> p = succ(pred), q;; p = q) { |
699 |
|
|
if (p == null || (item = p.item) != null) { |
700 |
dl |
1.1 |
nextNode = p; |
701 |
jsr166 |
1.114 |
E x = nextItem; |
702 |
dl |
1.1 |
nextItem = item; |
703 |
jsr166 |
1.114 |
return x; |
704 |
jsr166 |
1.48 |
} |
705 |
jsr166 |
1.114 |
// unlink deleted nodes |
706 |
|
|
if ((q = succ(p)) != null) |
707 |
dl |
1.123 |
NEXT.compareAndSet(pred, p, q); |
708 |
dl |
1.1 |
} |
709 |
|
|
} |
710 |
tim |
1.2 |
|
711 |
dl |
1.1 |
public void remove() { |
712 |
dl |
1.23 |
Node<E> l = lastRet; |
713 |
dl |
1.1 |
if (l == null) throw new IllegalStateException(); |
714 |
|
|
// rely on a future traversal to relink. |
715 |
jsr166 |
1.64 |
l.item = null; |
716 |
dl |
1.1 |
lastRet = null; |
717 |
|
|
} |
718 |
|
|
} |
719 |
|
|
|
720 |
|
|
/** |
721 |
jsr166 |
1.79 |
* Saves this queue to a stream (that is, serializes it). |
722 |
dl |
1.1 |
* |
723 |
jsr166 |
1.95 |
* @param s the stream |
724 |
jsr166 |
1.96 |
* @throws java.io.IOException if an I/O error occurs |
725 |
jsr166 |
1.48 |
* @serialData All of the elements (each an {@code E}) in |
726 |
dl |
1.1 |
* the proper order, followed by a null |
727 |
|
|
*/ |
728 |
|
|
private void writeObject(java.io.ObjectOutputStream s) |
729 |
|
|
throws java.io.IOException { |
730 |
|
|
|
731 |
|
|
// Write out any hidden stuff |
732 |
|
|
s.defaultWriteObject(); |
733 |
tim |
1.2 |
|
734 |
dl |
1.1 |
// Write out all elements in the proper order. |
735 |
jsr166 |
1.48 |
for (Node<E> p = first(); p != null; p = succ(p)) { |
736 |
jsr166 |
1.64 |
Object item = p.item; |
737 |
dl |
1.1 |
if (item != null) |
738 |
|
|
s.writeObject(item); |
739 |
|
|
} |
740 |
|
|
|
741 |
|
|
// Use trailing null as sentinel |
742 |
|
|
s.writeObject(null); |
743 |
|
|
} |
744 |
|
|
|
745 |
|
|
/** |
746 |
jsr166 |
1.79 |
* Reconstitutes this queue from a stream (that is, deserializes it). |
747 |
jsr166 |
1.95 |
* @param s the stream |
748 |
jsr166 |
1.96 |
* @throws ClassNotFoundException if the class of a serialized object |
749 |
|
|
* could not be found |
750 |
|
|
* @throws java.io.IOException if an I/O error occurs |
751 |
dl |
1.1 |
*/ |
752 |
|
|
private void readObject(java.io.ObjectInputStream s) |
753 |
|
|
throws java.io.IOException, ClassNotFoundException { |
754 |
tim |
1.2 |
s.defaultReadObject(); |
755 |
jsr166 |
1.55 |
|
756 |
|
|
// Read in elements until trailing null sentinel found |
757 |
|
|
Node<E> h = null, t = null; |
758 |
jsr166 |
1.104 |
for (Object item; (item = s.readObject()) != null; ) { |
759 |
jsr166 |
1.48 |
@SuppressWarnings("unchecked") |
760 |
jsr166 |
1.102 |
Node<E> newNode = newNode((E) item); |
761 |
jsr166 |
1.55 |
if (h == null) |
762 |
|
|
h = t = newNode; |
763 |
|
|
else { |
764 |
jsr166 |
1.125 |
NEXT.set(t, newNode); |
765 |
jsr166 |
1.55 |
t = newNode; |
766 |
|
|
} |
767 |
dl |
1.1 |
} |
768 |
jsr166 |
1.55 |
if (h == null) |
769 |
jsr166 |
1.102 |
h = t = newNode(null); |
770 |
jsr166 |
1.55 |
head = h; |
771 |
|
|
tail = t; |
772 |
|
|
} |
773 |
|
|
|
774 |
dl |
1.86 |
/** A customized variant of Spliterators.IteratorSpliterator */ |
775 |
jsr166 |
1.130 |
final class CLQSpliterator implements Spliterator<E> { |
776 |
dl |
1.89 |
static final int MAX_BATCH = 1 << 25; // max batch array size; |
777 |
dl |
1.82 |
Node<E> current; // current node; null until initialized |
778 |
|
|
int batch; // batch size for splits |
779 |
|
|
boolean exhausted; // true when no more nodes |
780 |
|
|
|
781 |
|
|
public Spliterator<E> trySplit() { |
782 |
dl |
1.89 |
Node<E> p; |
783 |
|
|
int b = batch; |
784 |
|
|
int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1; |
785 |
jsr166 |
1.87 |
if (!exhausted && |
786 |
jsr166 |
1.130 |
((p = current) != null || (p = first()) != null) && |
787 |
dl |
1.83 |
p.next != null) { |
788 |
dl |
1.92 |
Object[] a = new Object[n]; |
789 |
dl |
1.82 |
int i = 0; |
790 |
|
|
do { |
791 |
|
|
if ((a[i] = p.item) != null) |
792 |
|
|
++i; |
793 |
|
|
if (p == (p = p.next)) |
794 |
jsr166 |
1.130 |
p = first(); |
795 |
dl |
1.82 |
} while (p != null && i < n); |
796 |
|
|
if ((current = p) == null) |
797 |
|
|
exhausted = true; |
798 |
dl |
1.89 |
if (i > 0) { |
799 |
|
|
batch = i; |
800 |
|
|
return Spliterators.spliterator |
801 |
jsr166 |
1.121 |
(a, 0, i, (Spliterator.ORDERED | |
802 |
|
|
Spliterator.NONNULL | |
803 |
|
|
Spliterator.CONCURRENT)); |
804 |
dl |
1.89 |
} |
805 |
dl |
1.82 |
} |
806 |
|
|
return null; |
807 |
|
|
} |
808 |
|
|
|
809 |
dl |
1.90 |
public void forEachRemaining(Consumer<? super E> action) { |
810 |
dl |
1.82 |
Node<E> p; |
811 |
|
|
if (action == null) throw new NullPointerException(); |
812 |
|
|
if (!exhausted && |
813 |
jsr166 |
1.130 |
((p = current) != null || (p = first()) != null)) { |
814 |
dl |
1.82 |
exhausted = true; |
815 |
|
|
do { |
816 |
|
|
E e = p.item; |
817 |
|
|
if (p == (p = p.next)) |
818 |
jsr166 |
1.130 |
p = first(); |
819 |
dl |
1.82 |
if (e != null) |
820 |
|
|
action.accept(e); |
821 |
|
|
} while (p != null); |
822 |
|
|
} |
823 |
|
|
} |
824 |
|
|
|
825 |
|
|
public boolean tryAdvance(Consumer<? super E> action) { |
826 |
|
|
Node<E> p; |
827 |
|
|
if (action == null) throw new NullPointerException(); |
828 |
|
|
if (!exhausted && |
829 |
jsr166 |
1.130 |
((p = current) != null || (p = first()) != null)) { |
830 |
dl |
1.82 |
E e; |
831 |
|
|
do { |
832 |
|
|
e = p.item; |
833 |
|
|
if (p == (p = p.next)) |
834 |
jsr166 |
1.130 |
p = first(); |
835 |
dl |
1.82 |
} while (e == null && p != null); |
836 |
|
|
if ((current = p) == null) |
837 |
|
|
exhausted = true; |
838 |
|
|
if (e != null) { |
839 |
|
|
action.accept(e); |
840 |
|
|
return true; |
841 |
|
|
} |
842 |
|
|
} |
843 |
|
|
return false; |
844 |
|
|
} |
845 |
|
|
|
846 |
dl |
1.83 |
public long estimateSize() { return Long.MAX_VALUE; } |
847 |
|
|
|
848 |
dl |
1.82 |
public int characteristics() { |
849 |
|
|
return Spliterator.ORDERED | Spliterator.NONNULL | |
850 |
|
|
Spliterator.CONCURRENT; |
851 |
|
|
} |
852 |
|
|
} |
853 |
|
|
|
854 |
jsr166 |
1.97 |
/** |
855 |
|
|
* Returns a {@link Spliterator} over the elements in this queue. |
856 |
|
|
* |
857 |
jsr166 |
1.98 |
* <p>The returned spliterator is |
858 |
|
|
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
859 |
|
|
* |
860 |
jsr166 |
1.97 |
* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, |
861 |
|
|
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. |
862 |
|
|
* |
863 |
|
|
* @implNote |
864 |
|
|
* The {@code Spliterator} implements {@code trySplit} to permit limited |
865 |
|
|
* parallelism. |
866 |
|
|
* |
867 |
|
|
* @return a {@code Spliterator} over the elements in this queue |
868 |
|
|
* @since 1.8 |
869 |
|
|
*/ |
870 |
|
|
@Override |
871 |
dl |
1.85 |
public Spliterator<E> spliterator() { |
872 |
jsr166 |
1.130 |
return new CLQSpliterator(); |
873 |
dl |
1.82 |
} |
874 |
|
|
|
875 |
dl |
1.123 |
// VarHandle mechanics |
876 |
|
|
private static final VarHandle HEAD; |
877 |
|
|
private static final VarHandle TAIL; |
878 |
|
|
private static final VarHandle ITEM; |
879 |
|
|
private static final VarHandle NEXT; |
880 |
dl |
1.71 |
static { |
881 |
jsr166 |
1.48 |
try { |
882 |
dl |
1.123 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
883 |
|
|
HEAD = l.findVarHandle(ConcurrentLinkedQueue.class, "head", |
884 |
|
|
Node.class); |
885 |
|
|
TAIL = l.findVarHandle(ConcurrentLinkedQueue.class, "tail", |
886 |
|
|
Node.class); |
887 |
|
|
ITEM = l.findVarHandle(Node.class, "item", Object.class); |
888 |
|
|
NEXT = l.findVarHandle(Node.class, "next", Node.class); |
889 |
jsr166 |
1.108 |
} catch (ReflectiveOperationException e) { |
890 |
dl |
1.71 |
throw new Error(e); |
891 |
jsr166 |
1.48 |
} |
892 |
|
|
} |
893 |
dl |
1.1 |
} |